Conventionally, when a surface of a sample, such as a substrate or a wafer, is observed or subjected to material inspection, or is inspected or evaluated with respect to a structure or electric continuity of electric circuits formed thereon, it is known to use a surface inspection apparatus in which a defect in a surface of a sample is detected by emitting a charged particle beam (a primary charged particle beam), such as an electron beam, to the surface for scanning, detecting secondary charged particles emitted from the surface, producing image data from the detected secondary charged particles, and comparing the produced image data with the image data for each die (or chip). Such surface inspection apparatuses including one that utilizes a scanning electron microscope (SEM), and surface flattening apparatuses for flattening a surface of a sample, such as a substrate, exist as independent apparatuses and have been conventionally used.
In such a conventional SEM-based system as described above, and a system which simultaneously illuminates a wide area on a wafer, as a wafer under inspection is irradiated with an electron beam, the wafer is charged. The overcharged wafer Would cause distorted image data and obscure images. In addition, a normal pattern may be erroneously evaluated as defective.
Another grave problem in the prior art is damages to a sample. Specifically, when an electron beam is irradiated to a surface of a wafer, the surface is charged by the irradiated beam, permitting acquisition of an image representative of a potential contrast. However, the wafer may be charged in a different condition depending on insulating material, metal conductive materials, circuit resistance, and the like. Therefore, occasionally, an extreme potential difference may be produced on a boundary of patterns, resulting in failed acquisition of secondary electrons emitted from the wafer surface or in arc discharge.
The above-mentioned problems will be described in greater detail.
Secondary electron emission characteristics differ from one sample to another depending on the energy of an incident irradiated beam thereon and the characteristic of the sample surface. FIG. 1 is a graph showing an exemplary relationship between beam energy and secondary electron emission efficiency η when an insulating material is irradiated with an electron beam. With η larger than one, more electrons than incident electrons are emitted from the insulating material, and hence the surface of the insulating material is positively charged (Region P). On the other hand, with η smaller than one, the surface is negatively charged (Region Q). This may cause damages such as breakdown in some samples, the characteristics of which depend on the circuit configuration and layered structure thereon.
Specifically, when an insulating material is applied with an electric field equal to or larger than a breakdown strength (for example, 50-1000 kV/mn), the insulating material loses the insulating property to cause a breakdown, resulting in a current flowing therethrough. On the other hand, when an excessive amount of charge is accumulated on the insulating material, the field strength exceeds a breakdown voltage, thereby resulting in a breakdown. Also, once a breakdown occurs, an excessive current flows to break the circuit, and the insulating material may no longer restore its insulating property.
Further, when the magnitude of a beam current is decreased so as to minimize distortion of the image data due to the electric charge, an S/N ratio of a signal resulting from a secondary charged particle beam, which is emitted from the surface of the sample by emission of a primary charged particle beam, becomes undesirably low. This increases the possibility of false detection. The problem of lowering of the S/N ratio may be lessened and the possibility of false detection may be reduced, by effecting scanning at a plurality of times and conducting an averaging operation. However, the throughput in such an inspection apparatus becomes low.
Further, in order to detect fine defects, a large-current emission beam is required. For example, when it is assumed that the amount of signal required for determining a defect having a 2×2 pixel size of a CCD is 1, the amount of signal required for determining a defect having a 1×1 pixel size is 4. That is, for detecting a fine defect by using the same detector, the magnitude of a beam current must be increased so as to increase the amount of secondary electrons. However, when the magnitude of a beam current is increased, as is described above, the electric charge on the sample becomes high, thus increasing distortion of the image.
In order to solve the above-mentioned problems, the Applicant proposed, in Japanese Patent Application No. 2000-340651 (published as Japanese Patent Public Disclosure (Kokai) No. 2002-148227), a method in which a resistive film is coated on the surface of a sample before emission of a charged particle beam to the surface. In this method, however, it is difficult to form a thin resistive film having a uniform thickness on each sample. Further improvements are required to be made.
Although a technique of coating a resistive film or coat on the surface of the sample before inspection was proposed by the forgoing application, no proposals have been made with respect to an inspection apparatus which enables a series of operations such as surface flattening, resistive film coating and emission of a charged particle beam for inspection to be efficiently conducted. Conventional independent apparatuses, such as a flattening apparatus for flattening a surface of a sample, a cleaning apparatus for cleaning a sample, and a drying apparatus, are individually placed with a resistive film coating apparatus, and each operation is conducted by using these apparatuses. With this arrangement, however, it is difficult to efficiently conduct the above-mentioned series of operations. The reason for this is as follows. That is, each of the above independent apparatuses comprises a sample loading station and a loading/unloading robot. A sample conveyed by a conveyor apparatus for conveying a sample between the apparatuses is temporarily loaded on the sample loading station. The loading/unloading robot moves the sample from the sample loading station to a work position, i.e., a stage device, and removes the sample from the stage device. A plurality of such independent apparatuses are individually placed and a conveyor apparatus for conveying a sample between the independent apparatuses is further provided. This results in a complicated structure and an increase in overall size of a surface inspection apparatus. Further, the time required for conveying the sample is long and the throughput in the entire apparatus is low. Further, the possibility of contamination and oxidation of a surface of a sample increases, leading to deterioration of product quality.